Author’s previous self-built all-solar property north of Broome. A tidal lagoon runs into the Indian Ocean

Historically, many measurements were variably defined by a reigning monarch’s needs and tax collectors’ perceptions. The medieval ‘virgate’ was a taxable area of farmed land. One virgate could vary from 15-60 acres, depending on how long it was deemed necessary to plough, its presumed fertility, the king’s need for cash, etc.

Many measurements were like this until the standardisations essential for the beginnings (in 1793) of what was to become the Metric System. We can nowadays usually trust data based on physical quantities, but the solar industry to some extent perpetuates the ‘historical’ approach – and governments variously ‘define’ promises.

In 1994, Krauter and Hanistsch’s paper presented at that year’s Photovoltaic Energy Conversion Conference noted that: “Actual efficiencies of photovoltaic generators are often lower than predicted by [the industry’s] Standard Operating Conditions.”

Around 2001, a local book by and about its designer/owner (Michael Mobb’s) renewable technology house noted that he was surprised to find the solar input to be substantially lower than he’d calculated. Being then aware that my own (then) 1.5 kW system north of Broome, rarely produced more than 1.1 kW, I was intrigued shortly after by a doctoral thesis arguing that industry standard test conditions did not reflect real life use. (Check your own system and you’ll find much the same).

‘VIRGATE’ SOLAR RATINGS

A month or two’s research showed that the industry has, in effect, two sets of scales. One, Standard Operating Conditions (SoC), is for rating solar module output under laboratory conditions. It does so using procedures and assumptions that are not achievable in real use. The other, called Nominal Cell Operating Temperature NOCT), reveals what the products are likely to produce in actual usage.

That there are two such methods would not matter if the NOCT version was used for selling. But it’s not. The industry markets its wares on the non-realistic (SoC)  laboratory-based ratings. It does not conceal that from the technically astute: many modules have a sticker on the back setting out what they really develop, and likewise in its technical literature. All, however, is in units that even electricians will find hard to interpret; the average buyer will find it impossible.

The reality is that, depending on the solar regulator used, a solar module in typical RV use will produce only 70-80 percent of that seemingly claimed. That is confirmed by the industry’s own Nominal Operating Cell Temperature rating.

Buyers reasonably assume that, if a promoted product is claimed to generate (say) 100 watts it does just that, but what buyers receive is 20-30 percent less. On big systems the dollar difference is huge.

BUYING WATTS

What surprises even more is that few in the industry appear to feel it matters. If queried, an industry spin doctor explains that it stems from an era when most buyers understood that Standard Operating Condition’s methodology did not reflect reality, but that the results could be correlated with what really happens. (This is not uncommon in measuring technology – but not for promotion and selling).

Hardly anyone in the industry disputes that products not only do not, but cannot deliver that seemingly offered. It is simply argued that, because it’s gone on since 1970 or so, it’s become ‘standard industry practice’ so there’s no reason to change it now. Stealing has been ‘standard industry practice’ in the burglary trade for millennia – yet few outside that industry argue it’s acceptable.

Others (even an article in a magazine that should have known better) explain why there is that difference between claim and reality – as if that justifies the anomaly. Another response is that, as few buyers understand the units employed, using them loosely doesn’t matter. Even fewer know the definition of a kilogram – yet are unlikely to short-sell potatoes.

Those who understand volts, amps and watts get short-changed too, often without understanding how or why. Physics teachers, for example know, with plus-Talmudic certainty, that one amp times one volt is one watt. They may audit their energy needs (in watt hours/day), then order solar capacity accordingly, based on the reasonable (but false) assumption that solar capacity marketed as (say) 1000 watts actually produces 1000 watts – not the 700-800 watts that it does.

Solar reality is that in real life application, no commercial solar module will come within cooee of its Standard Operating Condition unless it is:

• On top of a high mountain near the equator on a very cold day around noon, and
• Exposed to the full sun for only a second or so at a time.

DIFFERENCES EXPLAINED

The major issues are with systems that have battery storage. As one watt is defi nable as one volt times one amp, a 120 watt solar module delivering a realistic charging/operating average of (say) 14 volts must by definition, produce 8.57 amps. But measure it, or just read what it says on the sticker on the back, and you’ll find it typically produces 7.1 amps (i.e. 99.4 watts). Here’s why:  For rating solar output, the industry draws two graphs: one of volts, the other of amps. The graphs are ‘overlaid’ and where their product peaks (under Standard Operating Conditions) becomes the rated output. This is usually about 17.0 volts and, at 7.1 amps equals 120.7 watts.

That module can, however, only deliver its rated power at that 17 or so volts: but 17- volt batteries are as rare as pigs that read the Karma Sutra. In any 12-volt battery system with a conventional PWM solar regulator, the energy between peak and the charging voltage is not accessible. It’s a silicon version of the pea under the thimble trick – now you have it, now you don’t.

Some of that ‘inaccessible’ current can be accessed via the more sophisticated multiple power point tracking (MPPT) solar regulators. This works a bit like a torque converter in a car – but juggling amps and volts to optimise watts. It reduces the mismatch losses, but some energy is lost in the MPPT device itself. By and large an MPPT regulator regains 10-15 percent.

HEAT LOSS

The second cause of output discrepancy is heat loss.

Solar modules dislike being heated: typically losing about five percent output for every 10°C increase in temperature. Manufacturers openly reveal this, but vendors typically claim this does not matter because the modules are rated at 25°C anyway. But that 25°C is not outside temperature: it’s that of the black solar cells sitting under glass in the sun!

The Nominal Operating Cell Temperature spells that out. It is measured  using the same test conditions as the Standard Operating Conditions but assumes 20 percent less sun and a minor cooling breeze, etc. It also reveals that at 25°C ambient, those cells are somewhere between 47°-49°C – so another 10 percent of the output is lost even at that.

As a result of all this:

• Buyers do not receive that which they may reasonably assume to have paid for.
• The practice results in resellers and installers being locked into a global marketing ploy not of their making, and over which they have no control. Yet were a Trade Practices action to be brought, it is the supplier and installer who must defend it.
• Advertising in reputable media includes unsustainable claims.
• Contracts are being drawn up and signed that specify system wattage that is then not delivered and cannot be delivered.

The supplier of my current 2.4 kW system openly agreed, when challenged, that it was very unlikely to produce more than 2.2 kW and it has not in more than three years. The proven and realistic Nominal Operating Cell Temperature rating is just fine. It should be adopted.

Category: Technology
Written: Sat 01 Nov 2014
Printed: November, 2014
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